Today, there is a world wide effort to develop displays that are flexible and rugged so they can conform to three-dimensional configurations and also be repeatedly flexed. Displays are being developed that possess the flexibility of a thin plastic sheet, paper or fabric, so that they can be draped, rolled up or folded like paper or cloth. This not only makes the display more portable and easier to carry, but expands its potential applications well beyond those of typical flat panel information displays: a display worn on the sleeve; the back of a bicyclists coat that shows changing direction signals; textile that changes its color or design, are but a few examples.
While the ability of an electrically addressable liquid crystal display to be flexible and deform like cloth or paper would be advantageous for any display technology, it is especially advantageous in the case of bistable reflective display technologies such as cholesteric liquid crystal displays. Cholesteric displays can be made very reflective such that they can be seen in bright daylight or in a dimly lit room without the aid of a backlight. They can be made to be bistable to conserve power even further so that they require power only when it is desired to refresh the image. Cholesteric liquid crystalline materials are unique in their optical and electro-optical features. Of principal significance, they can be tailored to Bragg reflect light at a pre-selected wavelength and bandwidth. This feature comes about because these materials posses a helical structure in which the liquid crystal (LC) director twists around a helical axis. The distance over which the director rotates 360 degrees is referred to as the pitch and is denoted by P. The reflection band of a cholesteric liquid crystal is centered at the wavelength, λO=0.5(ne+no)P and has the bandwidth, Δλ=(ne−no)P which is usually about 100 nm where ne and no are the extra-ordinary and ordinary refractive indices of the LC, respectively. The reflected light is circularly polarized with the same handedness as the helical structure of the LC. If the incident light is not polarized, it will be decomposed into two circularly polarized portions with opposite handedness and one of the portions reflected. The cholesteric material can be electrically switched to either one of two stable textures, planar or focal conic, or to a homeotropically aligned state if a suitably high electric field is maintained. In the planar texture the helical axis is oriented perpendicular to the substrate to Bragg reflect light in a selected wavelength band whereas in the focal conic texture it is oriented, on the average, parallel to the substrate so that the material is transparent to all wavelengths except for weak light scattering that is negligible on an adjacent dark background. These bistable structures can be electronically switched between each other at rapid rates on the order of milliseconds. Gray scale is also available in that only a portion of a pixel can be switched to the reflective state thereby controlling the reflective intensity.
The bistable cholesteric reflective display technology is ideal for displays on cloth or fabric that can be body worn as the bistability feature avoids the need for refreshing power and high reflectivity avoids the need for power-consuming backlights. These combined features can extend battery lifetimes from hours to months over displays that do not have these features. Reflective displays are also easily read in very bright sunlight where backlit displays are ineffective. Because of the high reflective brightness of a cholesteric display and its exceptional contrast, a cholesteric display can be easily read in a dimly lit room. The wide view angle offered by a cholesteric display allows several persons to see the display image at the same time from different positions. Cholesteric displays have several important electronic drive features that other bistable reflective technologies do not. Of extreme importance for addressing a matrix display of many pixels is the characteristic of a voltage threshold. A threshold voltage is essential for multiplexing a row/column matrix without the need of an expensive active matrix (transistor at each pixel). Bistability with a voltage threshold allows very high-resolution displays to be produced with low-cost passive matrix technology.
In addition to bistable cholesteric displays with liquid crystalline materials having a positive dielectric anisotropy, it is possible to fabricate a cholesteric display with liquid crystalline materials having a negative dielectric anisotropy as, for example, described in the U.S. Pat. No. 3,680,950 to Haas et al. or Pat. No. 5,200,845 to Crooker et al., incorporated herein by reference. These “negative materials,” like “positive” materials, are chiral nematic liquid crystals that are prepared from nematic materials that have been twisted into a helical molecular arrangement by the addition of a chiral compound or collection of chiral compounds. The negative and positive materials are prepared from nematic liquid crystals with either a negative or positive dielectric anisotropy respectively.
Negative type cholesteric displays can operate in a bistable mode where the material is switched into the stable planar (e.g., color reflective) texture with an AC pulse or into the stable focal conic (e.g., transparent) texture with a DC pulse as described by U.S. Pat. No. 3,680,950. There are other modes of operation such as has been disclosed by Crooker where a droplet dispersion of negative cholesteric materials is switched into the planar, color reflective texture with an applied electric field, but relaxes back into a transparent texture when the field is removed.
Some cholesteric materials possess a dielectric anisotropy that can be negative under an applied electric field of one frequency but positive at another frequency. This feature can be used to drive a bistable display using a dual frequency drive scheme as described in U.S. Pat. No. 6,320,563.
Another important feature of cholesteric materials is that the layers reflecting red, green, and blue (RGB) colors as well as IR night vision can be stacked (layered) on top of each other without optically interfering with each other. This makes maximum use of the display surface for reflection and hence brightness. This feature is not held by traditional displays where the display is broken into pixels of different colors and only one third of the incident light is reflected. Using all available light is important for observing a reflective display in a dimly lit room without a backlight or frontlight. Gray scale capability allows stacked RGB, high-resolution displays with full-color capability where as many as 4096 colors have been demonstrated. Because a cholesteric display cell does not require polarizers, low cost birefringent plastic substrates such as PET can be used. Other features, such as wide viewing-angles and wide operating temperature ranges as well as fast response times make the cholesteric bistable reflective technology, the technology of choice for many low power applications.
Cholesteric liquid crystal displays on flexible plastic substrates have been reported by Minolta Co. Ltd. and by the present assignee, Kent Displays, Inc., involving two plastic substrates filled with cholesteric liquid crystal materials (Society for Information Display Proceedings, 1998, pp 897-900 and 51-54 respectively). While the substrates are flexible the assembled displays are much less flexible because of the lamination of two substrates together. Kent Displays Inc. has also found that greater flexibility can be achieved if only one substrate is used and the display materials are coated, printed on the substrate or on a release liner for transfer to a flexible substrate as disclosed in Patent Application No.: PCT/US2005/003144, entitled “Liquid Crystal Display Films,” filed Jan. 28, 2005, which is incorporated herein by reference in its entirety.
Cholesteric liquid crystals are made suitable for standard coating and printing techniques by use of polymer droplet dispersions. As droplet dispersions, the materials are made insensitive to pressure and shear such that an image on a bistable cholesteric display is not readily erased by flexing the substrate. Recently Stephenson et al. at Kodak fabricated flexible bistable reflective displays with polymer dispersions of cholesteric liquid crystals on a single transparent plastic substrate using photographic methods (U.S. Patent Application Publications: US 2003/0202136 A1 and US 2004/0032545 A1). This process involves a sequence of depositions on transparent polyester plastic whereby the end product is a display where the images are viewed through the substrate. Such a process requires substrate materials that are transparent such as a clear plastic sheet.
More recently, bistable cholesteric displays have been disclosed by the present assignee, Kent Displays, Inc., that do not require a transparent substrate, making available a broader range of substrate materials such as fabrics made of fibers that can be deformed not only by bending or being rolled up, but also being draped or folded as disclosed in U.S. application Ser. No. 11/006,100, filed Dec. 7, 2004, which is incorporated herein by reference in its entirety. These added features offer many advantages and open up new display applications. This brings to the marketplace a display that has the physical deformability of fabric so that it can be an integral part of clothing and have the feel and appearance of cloth in that it can be draped and has folds. Being electrically addressable these liquid crystal displays have the physical deformability or drapability of textile or cloth, which brings advantages in manufacturing where the display including the electrodes is made of organic materials and are coated or printed on the substrate. Conducting polymers are used instead of the traditional inorganic materials such as indium tin oxide, ITO, for the electrodes. On some fabrics, preparation layers are used to color, smooth or planarize the surface, adjust the resistivity, index match and other features. Polymer dispersions of cholesteric liquid crystals can be made from a wide variety of different methods as is suitable for various manufacturing processes or display function. This is a substantial advance in addressable liquid crystal displays wherein, by forming the displays on or integrally with a drapable substrate, the display itself is drapable. Such substrates include textiles or fabrics made of natural or man-made fibers such as cloth or paper, as well as non-fibrous materials such as flexible or even drapable polymeric sheets or films. The substrate need not be transparent. With deformable substrates, cholesteric or other liquid crystal displays are made flexible, rugged and can even be sewn into or onto clothing to provide a wearable display. In fact, the display itself can form the material used to make the clothing or other fabric construct. A display with the drapability of cloth provides a new dimension to the display technology enabling display applications that were not possible before such as conforming to three-dimensional structures or flexing and folding with the garment containing the display. To this end, the displays are operatively deformable, meaning that they will function even though they are or have been deformed. In preferred applications, the displays are operatively drapable such that they can have folds and possess a measurable drape coefficient. The formability of a fabric can be defined as its ability to re-form from a two-dimensional shape to a simple or complex three-dimensional shape.
A key difficulty with these new fabric substrates is the means of connecting the displays to their drive electronics, both electrically and mechanically. Displays on rigid (glass) or flexible (plastic) substrates are typically electrically connected to their drive electronics using flex connectors, conductive elastomer or metal pins. Mechanical attachment is provided to the printed circuit board holding the drive electronics by a bezel or adhesion. These mechanical and electrical connection approaches are all undesirable for a flexible display because they require rigidity to be added to the display at the point of connection. What is required is a means to secure the display in place and to provide electrical contacts without compromising the flexibility. Additionally, it must be possible to make and remove the connections both quickly and easily.
This invention features a new interconnect that provides electrical connection of flexible displays and electrical components connected thereto to the associated electronics. The interconnect of the present invention extends the basic concept of a zipper that has been used for years to quickly and easily mechanically connect pieces of fabric, into the new application of flexible liquid crystal displays.